1. Field of the Invention
This invention relates to a process for increasing the octane number while simultaneously reducing the sulfur content of olefinic gasolines derived from cracking processes, specifically catalytic cracking processes. The process employs a noble metal containing, high SiO.sub.2 /Al.sub.2 O.sub.3 ratio, large pore zeolite catalyst.
2. Discussion of the Prior Art
New regulations requiring reduction of lead in gasoline will lead to the need for higher average gasoline pool octanes. In addition, there is likely to be continued interest in reducing sulfur oxide (SOx) emissions, especially as gasolines derived from fluidized catalytic cracking (FCC) processes are integrated more into the unleaded gasoline pools for use in automobiles equipped with catalytic converters.
The possibility of catalytically reforming FCC naphtha to upgrade a gasoline pool was considered by L. A. Gerritsen, "Catalytic Reforming of FCC Naphtha for Production of Lead-Free Gasoline", Ketjen Symposium, Amsterdam, 1984, the entire disclosure of which is herein incorporated by reference. Such prior art disclosed the processing of a FCC naphtha fraction over a bimetallic Pt-Re catalyst. It was indicated that higher severity and increased throughput conditions of the process resulted in a deterioration of the cycle length of the catalyst in the reformer. As a consequence, the prior art recognized the need to replace conventional catalysts with more stable catalysts.
Many crystalline silicate zeolites are more known to the prior art. However, direct reforming of the olefinic gasolines derived from catalytic cracking, i.e., such as FCC or thermofor catalytic cracking (TCC), of gas oils leads to rapid aging of conventional reforming catalysts due to the relatively high sulfur content (0.05 to 0.5 wt. %) of these gasolines. The olefinic composition of these gasolines also leads to relatively high hydrogen consumptions and corresponding exotherms during the desulfurization necessary prior to reforming with conventional catalysts.
Thus, conventional catalysts, such as those disclosed in U.S. Pat. Nos. 3,293,192; 3,493,519; 3,591,488; 3,691,099; 4,218,307; 3,308,069; 3,402,996; and 4,191,638, the disclosures of each of which are herein expressly incorporated by reference, show the prior art attempts to achieve novel catalysts having desired properties or specialized utilities.
Certain hydrothermally stable catalysts, such as those taught in U.S. Pat. No. 3,493,519, employ an ammonium-Y crystalline aluminosilicate which is calcined in the presence of rapidly flowing steam. The resultant steamed product is base-exchanged with an ammonium salt and treated with a chelating agent capable of combining with aluminum at pH between about 7 and 9. These aluminum-deficient catalysts are reported to exhibit enormously high activity (alpha value).
Other treatments of synthetic faujasite (NH.sub.4 Y) prepared by ammonium ion-exchange of sodium faujasite are reported in U.S. Pat. No. 3,591,488. These steamed zeolites, after heat treatment, are base-exchanged with cations, such as ammonium ion, and/or metal ions selected from the following groups of the Periodic Table: Groups II-A, I-B to VII-B, VIII, and the rare earth ions with atomic numbers 51 to 71, such as the following metal ions: Mg, Ca, Sr, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, W, Re, Os, Ir, Pt, Au and Hg, and preferably those ions in Groups II-A, VIII and the rare earths. It has been reported that the use of this procedure removes nearly all alkali metal cations which were present prior to the steam treatment. A final zeolite product having an alkali metal content below about 0.5 wt %, preferably below about 0.2 wt %, is reported. By utilization of the steam treatment procedure, from 0 up to about 98% or more of the original alumina present in the crystalline aluminosilicates may be abstracted. The resultant products had silica-to-alumina mole ratios typically greater than 5 to 10, depending on the nature of the zeolite, preferably greater than 20, and more preferably greater than about 50. However, high silica-to-alumina ratios greater than these values are not disclosed in the prior art.
Additional hydrocarbon conversion processes and catalysts are disclosed in U.S. Pat. Nos. 4,021,331; 4,419,220; and 4,456,527, the disclosures of each of which are herein expressly incorporated by reference.
The problem of sulfur contamination of catalysts has been generally recognized in the prior art, as taught, for example, in U.S. Pat. No. 4,456,527. However, the prior art approached the catalyst contamination problem by employing separate sulfur removal steps to reduce the sulfur content below 500 parts per billion (ppb), preferably less than 250 ppb, more preferably less than 100 ppb, and most preferably less than 50 ppb.
Thus, although the prior art recognized the problems of catalyst contamination associated with high sulfur-containing feedstocks, none of these prior art attempts has permitted direct reforming of an olefinic gasoline derived from FCC or TCC catalytic cracking of gas oils, in which rapid aging of the reforming catalyst due to the relatively high sulfur content of these gasolines is minimized or avoided.